Solar-powered lighting devices are dependent on energy stored in a battery during periods of available solar-power. Typically, the battery is charged in the daytime and the lighting device is on during the night, powered by the battery. It is desirable that the stored energy in the battery will last from one time period of charging to the next. When the weather is poor for a long period the solar-energy may not be enough for charging the battery to a high enough level.
This problem has been addressed in prior art, such as in JP2008086109, which relates to a general system where a natural power plant charges a battery which drives a load. Weather forecast data is used to predict the future power generation by the power plant, and, on basis thereof, the future discharging of stored power is predicted. The load can be driven in at least two different power modes, including a normal power mode and a power saving mode, i.e. a low power mode. If it is anticipated that the discharging will empty the battery below a predetermined bottom level before the power plant will generate charging power again, then the power saving mode is set.
This way of operating the load is inaccurate and in the specific case according to this invention where the load is a lighting device, the general teachings of JP 2008086109 do not provide any detailed guidance.